1. Technical Field
The present invention relates generally to position determination and, in particular, concerns server-assisted position determination technology.
2. Description of the Related Art
Satellite position systems (SPS) determine the location of a receiver based on signals received from a plurality of satellites. In one example of an SPS, known as a Global Positioning System (GPS), receivers normally determine their position by computing relative times of arrival of signals transmitted simultaneously from a multiplicity of GPS satellites. These satellites transmit, as part of their message, both satellite positioning data as well as data on clock timing, so-called “ephemeris” data. The process of searching for and acquiring GPS signals, reading the ephemeris and GPS system data (ionospheric data, universal time coordinated (UTC) data, almanac data, health data) for a multiplicity of satellites is time consuming. In many cases, this lengthy processing time is unacceptable and, furthermore, greatly limits battery life in micro-miniaturized portable applications.
Another limitation of current GPS receivers is that successful operation is limited to situations in which multiple satellites are clearly in view, i.e., without obstructions, and a good quality antenna is properly positioned to receive such signals. As such, current GPS receivers normally are unusable in portable body-mounted applications in areas where there is significant foliage or building blockage (e.g., urban canyons), and in-building applications.
One known solution to this problem involves the use of a wireless telephone integrated with a GPS receiver. The term “wireless telephone” includes, but is not limited to, cellular telephones, personal communication systems (PCS) devices, analog wireless telephones, digital wireless telephones, and the like. Although examples may be provided of specific types of wireless telephones, the present invention is not limited to any specific form of wireless communication device. A wireless link is established between the mobile GPS unit, or client, and a server, which is sometimes referred to as a position determining entity (PDE). The mobile GPS receiver takes what is referred to as a snapshot of the available satellite signals. That is to say, the mobile GPS receiver records a short duration of signals from as many satellites as are available in its line of sight. As described above, obstructions often limit the number of satellites that can be detected by the mobile GPS receiver and may also degrade the quality of the signals received by the mobile GPS receiver. In the presence of such obstructions, the received satellite signals are likely to be attenuated and fragmented, due to the location of the mobile GPS receiver, which may be in close proximity to buildings or foliage, etc.
There are known techniques for using an SPS, such as GPS with a reference network. A plurality of reference receivers forward received GPS data to a central location, such as the PDE. The PDE also receives data from the mobile GPS receiver and analyzes the mobile GPS data with respect to GPS data from the reference receivers. As a result of this analysis, the PDE is able to determine the location of the mobile GPS receiver.
FIG. 1 is a functional block diagram illustrating existing technologies for wireless assisted location technologies. The system 10 comprises a plurality of reference stations 12-16 that include the necessary hardware to receive GPS satellite signals and transmit the received data via a communication link 18. In a typical implementation, the communication link 18 is a wide area network (WAN). A communication controller 20 receives the reference data and may multiplex the data for convenience in subsequent transmission. The communication controller 20 receives the various forms of reference data from the reference stations 12-16 and transmits the reference data to a position determining entity (PDE) 22 via a communication link 24. In a typical implementation, the communication link 18 is a WAN.
The reference stations 12-16, the communication link 18, and the communication controller 20 may be described as a wide area reference network (WARN) 26. In a typical implementation of the WARN 26, the reference stations 12-16 are widely distributed throughout a region in precisely known locations. FIG. 1 illustrates reference stations 1−N where N may vary based on the area of coverage. For example, the continental United States may have a dozen or more reference stations distributed throughout the country.
Although not illustrated in FIG. 1, each of the reference stations 12-16 includes an antenna, a GPS receiver, and network interface equipment. In general, the reference stations 12-16 comprise survey grade GPS receivers that are capable of transmitting and receiving messages through communication ports, such as an RS-232 serial port. One example of a GPS receiver capable of operating as a reference receiver (e.g., in the reference stations 12-16) is the NovAtel receiver, which generates both pseudorange and Doppler measurement data. This data, which is sometimes referred to as measurement output data, is supplied by virtually all commercial GPS receivers. In addition, some receivers, such as the NovAtel receiver, are also capable of generating GPS navigation message output data, in raw and/or pre-processed forms. Such receivers are commercially available and need not be described in greater detail herein.
The various reference stations 12-16 forward reference data to the PDE 22. The reference data includes such data as pseudorange and Doppler data from single-frequency or dual-frequency reference stations, raw GPS navigation message output data, and a one-pulse-per-second signal for time synchronization. In some circumstances, the reference stations 1216 may not support all of the reference data described above. However, the reference data described above are illustrative of typical reference data from the reference stations 12-16.
Also illustrated in FIG. 1 is a mobile GPS unit 30, which may typically be embodied in a combination wireless communication device (e.g., a cellular telephone or PCS device) combined with a GPS receiver. As noted above, the mobile GPS unit 30 generally does not receive satellite data of sufficient quantity and quality to enable independent accurate determination of the location of the mobile GPS unit 30. Instead of an independent position determination, the mobile GPS unit 30 transmits the fragmentary GPS data that it has received to a wireless communication controller 32. The wireless communication controller 32 may be a conventional base transceiver station (BTS) or the like. The wireless communication controller 32 transmits the fragmentary measurement data to the PDE 22 via a communication link 34. In a typical implementation, the communication link 34 may be a WAN.
The PDE 22 analyzes the GPS measurements (code phase and/or other measurements) data from the mobile GPS unit 30 with respect to the reference data provided by the WARN 26. For example, in one implementation, fragmented data samples from a quadrature receiver (i.e., I/Q data samples) are used to “pattern match” with a GPS navigation message data from reference stations (e.g., the reference stations 12-16). The concept of pattern matching signals from a GPS receiver with GPS signals from a reference station is known in the art and need not be described in greater detail herein. The PDE 22 can then use these time-stamped measurements, using known methods, to accurately determine the location of the mobile GPS unit 30. The PDE 22 may transmit the position data back to the mobile GPS unit 30 via the wireless communication controller 32. The process of position determination using a reference network is well known and need not be described in greater detail herein.
In operation, the raw data measurements and navigation message from each of the reference stations 12-16 in the WARN 26 is forwarded to the PDE 22 for processing with data received from the mobile GPS unit 30. On the basis of the complete (or nearly complete) reference data from the plurality of reference stations 12-16 and the data from the mobile GPS unit 30, the PDE 22 can determine the location of the mobile unit with a high degree of accuracy.
The difficulty with such an approach is the additional data load on the PDE 22. Therefore, it can be appreciated that there is a significant need for an apparatus and method that will efficiently determine the location of a mobile GPS unit based on reference data from the WARN. The present invention provides this, and other advantages, as will be apparent from the following detailed description and accompanying figures.